1. Field
The present invention relates generally to wireless communications, and more specifically to equalization of received signals in wireless communication devices.
2. Background
Communications systems are used for transmission of information from one device to another. Prior to transmission, information is encoded into a format suitable for transmission over a communication channel. The transmitted signal is distorted as it travels through the communication channel. The signal also experiences degradation from noise and interference acquired during transmission. An example of interference commonly encountered in band-limited channels is called inter-symbol interference (ISI). ISI occurs as a result of the spreading of a transmitted symbol pulse due to the dispersive nature of the channel, which results in an overlap of adjacent symbol pulses. Another example of interference is performance degradation internal to the receiver itself such as interference caused by noisy equalizer taps. The received signal is decoded and translated into the original pre-encoded form. Both the transmitter and receiver are designed to minimize the effects of channel imperfections and internal interference. For the purposes of this disclosure, interference or distortion due to channel imperfections, internal interference, or any combination thereof will be referred to generally as noise.
Various receiver designs may be implemented to compensate for noise caused by the transmitter and the channel. By way of example, an equalizer is a common choice for correcting ISI. An equalizer corrects for distortions and generates an estimate of the transmitted symbol. In the wireless environment, equalizers are required to handle time-varying channel conditions. Ideally, the response of the equalizer adjusts to changes in channel characteristics. Equalizers are generally complex, tending to increase the power consumption of a communication device and introduce internal interference. Equalization is a key aspect of any WCDMA downlink receiver. Due to the large bandwidth used for WCDMA communications, the frequency selective behavior of the wireless channel is a concern and must be compensated for at the receiver using equalization techniques.
Current equalizer implementations comprise an Finite Impulse Response (FIR) filter with chip-spaced complex taps, which are updated periodically. The equalizer taps are computed using a frequency domain algorithm, which is essentially a low complexity approximation of the true minimum mean squared error (MMSE) equalizer. Because the channel impulse response (CIR) and covariance estimates used to compute the equalizer taps are noisy due to estimation errors (even after filtering), the noise propagates to the equalizer taps and results in performance degradation. Regardless of the operating channel conditions, a fixed number of equalizer taps are always computed.
However, in a single path (equivalently, frequency flat) channel, just one equalizer tap is sufficient to mitigate the ISI and other distortion introduced by the wireless channel. In this case, the remaining equalizer taps merely act as a source of noise and degrade the Signal to Noise Ratio (SNR) at the output of the equalizer. The impact is especially visible in high geometry conditions, where the noisy equalizer taps become the dominant noise source. An improved equalizer design would reduce power consumption and its own internal interference, as well as provide optimum performance under various channel conditions.
There is therefore a need in the art for a method and apparatus to detect single path channel conditions and reduce the span (number of taps) of the equalizer in order to mitigate the performance degradation caused by noisy equalizer taps.